1. Field of the Invention
The present invention relates to a process for the photoelectrochemical etching of semiconductor electrodes. A permanent image capable of providing a visible display of differentially electroluminescing semiconductors is provided.
2. Background of the Art
Electroluminescence occurs in semiconductor materials which are capable of emitting visible or near visible radiation when an electrical current passes through the semiconductor. Photoluminescence can also occur in these materials. If external light is used to excite the semiconductor, a characteristic wavelength of light is emitted. These characteristic wavelengths vary amongst different photoluminescent semiconductors and can be varied in a single semiconductor by doping the material. The dopant will ordinarily cause a shift in the wavelength of radiation emitted by the material.
Amongst the various studies on the luminescence of photo-stimulated electroluminescent materials is "Luminescent Photoelectrochemical Cells", Streckert, H. H., Tong, J. and Ellis, A. B., J. Am. Chem. Soc., Vol. 104, No. 2, 1982, pp. 581-588. It is noted therein that the intensity of light emitted by electroluminescence varies directly with the applied voltage. The efficiency of charge transfer and good electrical contact at the surface is also noted as important in the efficiency of the process.
In U.S. patent application Ser. No. 480,471, filed on Mar. 30, 1983 it has been disclosed that a multicolor graded electroluminescent article may be produced. The article must comprise a layer of a first electroluminescent semiconductor, and over only a portion of the surface of said layer is a second electroluminescent semiconductor emitting a characteristic wavelength at least 50 nm different from the characteristic wavelength of the first semiconductor.
The process for manufacturing such graded semiconductor electrodes is disclosed in U.S. patent application Ser. No. 478,476, filed Mar. 24, 1983 now U.S. Pat. No. 4,543,511. One particularly useful process of forming such graded compositions is to diffuse one material (such as sulfur or selenium) into the surface of one photoconductor layer (such as cadmium selenide or cadmium sulfide, respectively). Resistive etching of the graded layers is shown in Ser. No. 480,471.